1. Field of the Invention
The present invention relates to a reflected light measuring apparatus and a reflected light measuring method which project measuring light onto an object to be measured and receive its reflected light for various sensing purposes.
2. Description of the Related Art
Various kinds of reflected light measuring apparatuses are known in the prior art which project measuring light onto an object to be measured and receive and analyze its reflected light for such purposes as color evaluation, surface condition detection, and other sensing operations.
FIG. 13 is a schematic diagram showing the basic configuration of such a reflected light measuring apparatus. The apparatus shown is constructed such that the measuring light emitted from a spot light source (spot-like light source) 10 such as a halogen lamp, xenon lamp, or LED is projected onto a specimen 20 to be measured, and its reflected light is directed through a light receiving lens 30 to a light receiving sensor 40 such as a photodiode or CCD. Here, rays of light emitted from the spot light source 10 are first passed through a beam regulating plate 101 for conversion into parallel rays of light which are projected onto the surface of the specimen 20 to be measured.
FIG. 14 is a schematic diagram showing in simplified form the configuration of a reflected light measuring apparatus employing a multidirectional illumination, unidirectional light receiving method which is used, for example, in a multi-angle calorimeter or the like. The basic configuration of this reflected light measuring apparatus is the same as that shown in FIG. 13, the only difference being that the measuring light is projected onto the specimen 20 from a plurality of angles.
The example shown in FIG. 14 uses two light illumination systems: a highlight illumination system 50 whose angle (indicated by θ1 in the figure) to the normal to the surface of the specimen 20 is relatively small, and a shade illumination system 60 whose angle (indicated by θ2 in the figure) to the normal to the specimen surface is relatively large. The highlight illumination system 50 and the shade illumination system 60 comprise spot light sources 500 and 600, beam regulating plates 501 and 601, and lenses 502 and 602, respectively, and are constructed so that parallel rays of light as the measuring light from the respective light sources can be projected onto the surface of the specimen 20 at angles θ1 and θ2, respectively.
Reflected light measuring apparatuses employing such a multidirectional illumination, unidirectional light receiving method are used for such purposes as sensing the surfaces of specimens whose reflected light intensities vary depending on the viewing angle, for example, in calorimeters that measure metallic paint or pearly paint colors used for automobiles, etc.
When the measuring light emitted from the spot light source 10 is projected as parallel rays of light onto the specimen 20 to be measured, as in the prior art shown in FIG. 13, the resulting reflection characteristics are as shown in FIG. 15 when the measuring area surface of the specimen 20 is a horizontal plane surface. That is, in FIG. 15, Q10, Q20, and Q30 show the reflection characteristics (for example, the light intensity distribution of light of specific wavelength) obtained when the measuring light rays P1, P2, and P3 as parallel rays of light are projected onto and reflected from arbitrarily taken points A, B, and C, respectively, on the measurement surface (horizontal measuring area surface) 20S of the specimen 20.
In the figure, h10, h20, and h30 indicate the specular reflections of the measuring light rays P1, P2, and P3 (that is, the reflected light rays are symmetric to the incident rays with respect to the normal to the measurement surface 20S). In this case, since the measurement surface 20S of the specimen 20 is a horizontal plane surface, the reflection characteristics Q10, Q20, and Q30 are the same (that is, the reflection characteristics are the same within the measuring area surface), and stable measurements can thus be accomplished without causing any particular measurement errors within the measuring area surface.
However, when the measuring area surface of the specimen 20 is a curved surface, if the measuring light is projected in the form of parallel rays of light, the angles of the reflected light rays (the angles of the specularly reflected light rays) differ from each other within the measuring area surface, and this causes measurement errors.
FIG. 16 shows the reflection characteristics when the measuring area surface 20S of the specimen 20 is a curved surface of radius “r”. As shown, when the measuring light rays P1, P2, and P3 as parallel rays of light are projected onto the arbitrarily taken points A, B, and C on the measurement surface 20S, the specularly reflected light rays from the respective points are h11, h21, and h31 shown by solid lines in the figure, exhibiting the reflection characteristics Q11, Q21, and Q31 based on the specularly reflected light rays h11, h21, and h31, respectively.
Here, when the point B is considered as the reference point of the normal, since the measurement surface 20S is a curved surface, the reflection characteristics Q11 and Q31 at the points A and C are displaced from the reflection characteristics Q10 and Q30 (indicated by dotted lines in the figure) obtained when the measurement surface 20S is a horizontal plane surface, and these displacements show up as measurement errors.
Generally, a reflection characteristic is a combination of the characteristic due to diffused reflection that has no angle dependence and the characteristic due to specular reflection. In each of the above reflection characteristics Q10 to Q31, the gently sloping portion is the characteristic due to diffused reflection, and the sharply pointed portion in the center is the characteristic due to specular reflection. It therefore follows that as the direction in which the reflected light to be sensed is received becomes closer to the direction of the specular reflection, a greater difference is caused in the reflection characteristic even by a slight change in angle. This will be explained with reference to FIG. 17.
Suppose that the measuring light rays P1, P2, and P3 as parallel rays of light are projected as shown in the figure onto the arbitrarily taken points A, B, and C on the measurement surface 20S, and that the measuring reflected light rays to be received for sensing the reflection characteristics at the respective points are defined as shown by h1s, h2s, and h3s in the figure. Here, the specular reflections at the points A, B, and C, due to the incidence of the measuring light rays P1, P2, and P3, are as shown by h11, h21, and h31, respectively, the directions of which are relatively close to those of the measuring reflected light rays h1s, h2s, and h3s. 
In this case, the reflection characteristic Q11 at the point A exhibits a characteristic more or less straightened up toward the plumb line compared with the reflection characteristic Q10 obtained when the measurement surface 20S is a horizontal plane surface, while on the other hand, the reflection characteristic Q31 at the point C exhibits a characteristic more or less tilted toward the horizontal line compared with the horizontal plane reflection characteristic Q30.
Here, because the directions of the specular reflections h11, h21, and h31 are close to those of the measuring reflected light rays h1s, h2s, and h3s, the measuring reflected light ray h1s at the point A exhibits a large reflection characteristic value (the value at the intersection d1 where h1s intersects a sloping side of the sharply pointed portion of the reflection characteristic Q11), whereas the measuring reflected light ray h3s at the point C exhibits a small reflection characteristic value (the value at the intersection d2 where h3s intersects the rising section of the sharply pointed portion of the reflection characteristic Q31).
As a result, although the angle difference between the points A and C is very small, a large difference equivalent to the difference Δd between the reflection characteristic values at the intersections d1 and d2 occurs between the reflection characteristics, resulting in the problem that the measurement error within the measuring area surface appreciably increases.
Another problem is that, in the reflection light measuring apparatus shown in FIG. 14, it is difficult to project the parallel rays of light over the entire measuring area surface with the highlight illumination system 50 whose angle θ1 to the normal to the surface of the specimen 20 is relatively small.
That is, when the angle θ1 to the normal to the specimen surface is relatively small, the amount of illumination (the illumination area) necessary to illuminate the entire measuring area surface increases, making it difficult to illuminate the entire measuring area with the parallel rays of light. By contrast, with the shade illumination system 60 whose angle θ2 to the normal to the specimen surface is relatively large, since the measuring light is incident at a large angle on the measuring area surface, the entire measuring area surface can be illuminated with a relatively small amount of illumination, and it is therefore easy to ensure the parallelism of the light rays; with the highlight illumination system 50, on the other hand, it is not easy to ensure the parallelism of the light rays, the resulting problem being that a large-size optical system becomes necessary if the parallelism is to be ensured.
The reflection characteristic diagrams of FIGS. 15 to 17 assume that the measuring light rays P1, P2, and P3 incident on the measurement surface 20S are parallel rays of light, but if the parallelism is not ensured, variations will occur in the angle of specular reflection, which can incur another error.